Production of metals



April 15, 1958' R. s. HOOD 2,830,940

PRODUCTION OF METALS I Filed March 28, 1952 2 Sheets-Sheet l TiCl4 p y Ti C14 Vaporizer 24 FIG. I

Resevoir BubbleS INVENTOR. RALPH 5. HOOD ATTORNEY 'April 15, 1958 Filed March 28, 1952 R. s. HOOD PRODUCTION OF METALS 2 Sheets-Sheet 2 Frozen Elec'l'rolyfe FIG. 6

I'Ti Fines BY 'filecir y Mefal INVENTOR .RALPH S. HOOD ATTORNEY 2,330,940 Patented Apr. 15, 1958 hice PRODUCTION OF METALS Ralph 8. Hood, Marblehead, Mass., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware Application March 28, 1952, Serial No. 279,065 3 Claims; (Cl. 204--64) This invention relates to the production of metals and more particularly to the production of titanium in an electrolytic cell. i

A principal object of the present invention is to provide an improved process and apparatus for producing titanium in an electrolytic cell with a high power efficiency.

Still another object of theinvention is to provide such a process and apparatus which permit the use of the principle of bipolar cell operation with a consequent over-all power eificiency.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises theprocess involving the several steps and the relation and the order of one or more of such steps with respect to each of the others; and the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

Fig. l is a diagrammatic, schematic plan view of one electrolysis cell embodying the invention;

Fig. 2 is a diagrammatic, sectional view of the cell of Fig. 1 taken along the line 2-2;

Fig. 3 is an enlarged sectional view of a portion of the cell of Fig. 1 taken along the line 3-3;

. "Fig. 4 is a view similar to Fig. 3 showing type of cathode.

Fig.-5 is a-sectional view of the taken along the line 55; and

Fig. 6 is an enlargedyiew of a portion of Fig. 4.

in the production of titaniumby'electrolytic techniques, it is desirable that the titanium manufacture be achieved in each electrolytioxcell at the highest possible rate with the highest possible power efllciency. In. the present inven- ,tion these objectives are achieved by a number of features thereof. The cell preferably employs an electrolyte which comprises a molten mixture of halides and particularly the chlorides from the group consisting of the alkali metal chlorides and the alkali earth metal chlorides. To this electrolyte there is fed titanium tetrachloride, and the cell is operated at a sufiicientlyhigh voltage so that the cathode potential is above the discharge potential of one of the alkali metal or alkali earth metal ions. In this manner either the alkali metal or the alkali earth metal is liberated at the cathode. For convenience, the alkali metal or alkali earth metal liberated at the cathodes is sometimes hereinafter referred to as the electrolyte metal. Thiselectrolyte metal serves as a reducing agent which can reduce the titanium tetrachloride in the cell either to titanium metal or to a lower chloride.

Inany particular case it is diflicultto state whether or not the titanium tetrachloride is completely reduced by an alternative cathode of Fig. 4

the alkali metal or alkali earth metal to titanium metal, or whether it is only reduced to a lower chloride Which has a higher solubility in the electrolyte and which may be further reduced to titanium from solution. In either case much higher production rates are achieved since the electrolyte metal will be completely used up either by forming a more soluble lower chloride or by reducing a titanium chloride to titanium.

Titanium tetrachloride is preferably introduced into the electrolytic bath at a point below the cathode so that it rises in the electrolyte in the form of fine bubbles which pass upwardly into the electrolysis zone between the anode and cathode of the cell. 1 These rising bubbles of titanium tetrachloride serve several important functions. They act as a gas lift which accentuates the gas lift caused by the rising chlorine bubbles from the anode, and thus they cause an upward travel of the electrolyte in the electrolysis zone between the anode and cathode. The cell is preferably arranged so that the electrolyte travels downwardly in a settling zone at a point removed from the space between the anode and the cathode, and is then recirculated upwardly into the anode-cathode space.

The present invention permits utilization of the bi polar principle of electrolytic cell operation with a relatively simple cell construction. This is due to the fact that the reaction between titanium tetrachloride and the electrolyte metal (i. e., alkali metal or alkali earth metal) is highly exothermic. Consequently, considerable heat is generated within the electrolyte due to the presence of this exothermic reaction. Advantage is taken of this excess heat by providing cooling of the salt adjacent the ends of the bipolar plates, this frozen salt serving as an excellent insulation to completely isolate the various pairs of anodes and cathodes. The presence of this extra heat also permits the use of refractory materials in the cell as insulation barriers which are protected by frozen salt layers so that they are not attacked by the fused electrolyte. An additional advantage of the excess heat resides in the freezing of a protective salt layer over any metallic portions of the cathode which extend into the top of the cell where they might otherwise be attacked by chlorine vapor at the high temperature of the cell. The frozen salt can also be used as electrical insulation along the sides and bottom of the cell when these sides or bottom are made'of conducting materials such as carbon, metal and the like.

Referring now to Figs. 1, 2, and 3 there is shown one embodiment of the invention as applied to a bipolar cell. In this cell an anode and a cathode are positioned at opposite ends of the cell, and a plurality of bipolar elements are spaced along the cell. During the application of a voltage between the anode and cathode a lesser voltage is applied between the bipolar elements so that there are a plurality of electrolysis zones between the various bipolar elements. One side of each of the bipolar elements acts as an anode, while the other side thereof acts as a cathode. This cell is preferably so arranged that each electrolysis zone between each anode and cathode has a short dimension along the cell so as to cut down the IR drop in the electrolyte.

Titanium tetrachloride is preferably introduced into the cell adjacent the bottom of each cathode so as to cause an upward flow of the electrolyte in each electrolysis zone due to the gas lift of the titanium tetrachloride gas bubbles. The electrolyte flows upwardly Within each electrolysis zone, laterally at the top thereof, and then travels downwardly in an enlarged settling zone which is spaced to one side of, and outside of, each electrolysis zone. The downward flow of the electrolyte is in the settling zone where the relatively large particles of titanium settle out of the electrolyte stream while the titanium fines carried in thestream are recirculated back into the bottom of the electrolysis zone.

In Figs. 1 through 3 the electrolysis cell is generally indicated at Ill and comprises a main anode 12 at one end of the cell, and a main cathode 14 at the other end of the cell. Positioned between the main anode and main cathode is a plurality of bipolar elements, generally indicated at 15. Each of these bipolar elements comprises an anode 16 and a cathode 18. As illustrated, there are three of these bipolar elements 15, the electrolysis zones 20 between each of these elements having a relatively short dimension along the path of cur rent travel through the cell.

In a preferred embodiment of the invention each electrolysis zone is preferably on the order of less than one inch in thickness so as to provide for relatively smaller power loss in the cell. This zone is also preferably free of any mechanical restrictions to current flow between the anode and cathode. Each electrolysis zone 20 is in open communication at one end thereof with a settling zone 22 which is of a considerably larger size so as to provide a relatively slow flow of electrolyte downwardly therethrough. For electrically isolating each settling zone 22 from the adjacent settling zone, there is provided a plurality of insulating barriers 24. For removing the larger titanium particles which settle downwardly in the electrolyte in each settling zone 22, there is provided a pipe 26 which leads'to a filter 28.

Titanium tetrachloride is introduced into each electrolysis zone 20 between the anode and cathode defining this electrolysis zone by means of a plurality of pipes 30. In a preferred embodiment each pipe 30 is connected to a common manifold 31 which leads from a titanium tetrachloride vaporizer 32 which, in turn, is supplied from a titanium tetrachloride storage tank 33. Each pipe 30 is preferably positioned at the bottom of the corresponding cathode 14 or 18 (see Fig. 3) so that titanium tetrachloride bubbles which escape from the pipe 30 travel upwardly in the electrolyte adjacent the surface of each cathode 14 or 18. Excess titanium tetrachloride and chlorine gas are removed from the cell through a pipe 35 to a condenser which permits recovery of the titanium tetrachloride with recycling of the titanium tetrachloride to supply 33.

Electrolyte 34 is preferably introduced into cell 10 by means of pipes 36 from a supply 38 of the electrolyte. In a preferred embodiment, the electrolyte 34 is given a transverse velocity as it enters the cell through the pipe 36 so as to aid in achieving the transverse circulation of the electrolyte in the electrolysis zone 20. Electrolyte is pumped from the filter 28 by means of a pump 40 through a pipe 42 to the supply 38. In one preferred embodiment of the invention, the level of electrolyte 34 in the reservoir 38 is maintained higher than thelevel of electrolyte in the cell 10 so that a continuous feed of electrolyte to the cell is achieved by this difference in pressure.

For isolating the various electrolysis zones 20 from each other, there is preferablv provided an insulating member 44 positioned below each anode on each bipolar element 15. This is particularly illustrated in Fig. 3 where these insulating members 44 are shown as comprising refractory materials within which cooling pipes 46 are provided. The purpose of these cooling pipes 46 is to freeze a layer of electrolyte 3411 on the surface of the insulating members 44 so as to protect the insulating members 44 from attack by the electrolyte. Similar pipes'46 are also preferably provided at the top of each cathode 18 so as to freeze a layer of salt thereon to protect the. tops of these cathodes (which are metallic) against attack by chlorine. The cooling medium may comprisea liquid metal such as sodium-potassium or may be a fluid such as water or oil. Cooled insulating members, similar to those illustrated at 44, are also provided ad- .4 jacent the ends of the bipolar elements to serve as insulators adjacent these ends.

In a preferred embodiment of the invention, the walls of the cell 10 are of a suitable material such as carbon, metal or the like which are cooled by cooling coils (not shown) so as to freeze a layer 34a of electrolyte adjacent the surface of these walls. This layer of electrolyte is shown in Figs. 1 and 3 and serves as an adequate in sulation, due to its low conductivity, and prevents short ing of the cell. Equally, these walls can be made of refractories which are protected from molten salt attack by freezing a layer of salt thereon. The anodes 12 and 16 are preferably formed of elemental carbon, such as carbon, graphite, or mixtures of carbon and graphite, while the cathodes 14 and 18 may be formed of metals, such as titanium, molybdenum, and the like. Suitable thermal insulation (not shown) is provided on the cell and related equipment to prevent undue heat losses. The pipes 36 are made sufficiently long so that the voltage drop in the salt carried thereby is larger than the voltage drop between the bipolar electrodes to permit the cell to operate as a bipolar cell. If the filters 28 are electrically connected, the same applies to pipes 26 and 42. To the extent that the pipes 36, 30, 26 and 42 are formed of metal, they must include suitable insulation or be sufficiently long to prevent their acting as low resistance shunts to the cell. 7

In the operation of the cell the circulation of the electrolyte (shown best in Fig. 2) is achieved by three cooperating forces. The first of these is the chlorine gas lift resulting from chlorine generated at each anode. The second is the titanium tetrachloride gas lift resulting from the titanium tetrachloride introduced below each cathode. The third is the flow of electrolyte transversely of each electrolysis zone due to the introduction of electrolyte through the pipes 36. These forces all cooperate to give a counterclockwise rotation of the electrolyte as shown by the arrows in Fig. 2.

The introduction of titanium tetrachloride adjacent the bottom of each cathode in the bipolar cell, described above, has the very definite advantage that the exothermic reaction of the titanium tetrachloride vapors with electrolyte metal generated at each cathode furnishes extra heat to the cell which provides large excesses of heat over that necessary to keep the electrolyte molten. As a result of this large excess of heat, the walls of the cell may be chilled to permit freezing of the electrolyte, thus furnishing protection to the walls of the cell and providing adequate insulation between the various electrolysis zones.

In one preferred embodiment of the invention the cell is operated under the conditions set forth in Example I.

Example. I

Electrolyte composition:

Lithium chloride weight percent" 40 Potassium chloride do.... 60 Temperature of electrolyte C 475 to 550 Voltage volts 6 Cathode to anode spacing inch Cathode current density amps./in. 7.15 TiCl, fed to cell grams/hour 5200 Titanium production do In the preferred embodiment of Example I, which includes potassium chloride in the electrolyte, the top portion of the cell is maintained at approximately 525 C. to 550 C. so as to prevent the formation of a crust which might plug up the chlorine escape line 35. As described more fully in the copending application of Di Pietro Serial No. 279,160, filed on even date herewith, this crust is believed to be due to the formation of a compound between titanium tetrachloride and potassium chloride. This compound is considered to be unstable at temperatures over about 450 C. to 500 C. and elimination of this crust has been successfully accomplished by maintaining a space immediately above the bath at a temperature somewhat above 450 C. to 500 C. A separate heater (not shown) may be employed for maintaining the desired high temperature at the top of the cell. The total heat balance in the system, however, may be such that this additional heating is not essential, since considerable heat is generated within the cell by the exothermic reaction of the alkali metal or alkali earth metal with the titanium tetrachloride.

The titanium tetrachloride flow into the space forming each electrolysis zone 20 is preferably maintained sufficiently high so as to obtain adequate circulation of the electrolyte upwardly within the zone 20. As explained previously, this circulation is augmented by the gas lift of the chlorine bubbles rising up the surface of the anodes 12 and 16. The speed of circulation of the electrolyte is naturally a function of the height of the zone 20 as Well as the amount of chlorine and/or other gas rising in this zone 20. In general, it is desired that sufficient circulation be obtained so that all particles of titanium having a size less than about 300 microns be maintained in suspension in the electrolyte, and be carried back into the zone 20. However, other considerations, such as heat balance and the necessity for carefully controlling the electrolyte concentration, may dictate different circulation velocities. All titanium particles having a size greater than about 300 microns may be allowed to settle into the pipe 26 at the bottom of the cell where they may be periodically removed to the filter 28. The titanium tetrachloride gas flow into the cell is made sufiicient to move the electrolyte upwardly into the zone 20 with a velocity in excess of about 10 cm. per second. With this velocity the conditions for separation of larger titanium particles from the electrolyte stream are excellent. Since the smaller titanium particles are readily carried in suspension in the electrolyte stream they are readily circulated into the zone 20 while the larger titanium particles, due to their higher mass, readily settle out of the electrolyte stream and collect in the pipe 26 at the bottom of the cell.

The electrolyte and titanium particles in the pipe 26 are fed to the filters 28 where gross quantities of the salt are removed from the titanium particles and the salt is returned to the cell. The remainder of the electrolyte salt may then be removed from the titanium by vacuumdistilling or by washing with a low-melting-point salt, such as aluminum trichloride, as described more fully in the above identified Findlay et a1. application, Serial No. 252,564. The thus purified titanium powder is then preferably vacuum-distilled to remove the aluminum trichloride, and may be consolidated in accordance with usual techniques, such as by melting in an arc furnace to form an ingot.

In connection with the operation of the cell, it is extremely important that all oxides and nitrides be kept out of the electrolyte and that no air be permitted to contact the electrolyte during the operation of the cell. If there are any impurities in the salts comprising the electrolyte, these salts should be purified by known techniques prior to production of titanium in the cell.

In still other embodiments of the invention the conditions of operation of the cell are preferably the same as described in connection with the discussion of Example I. However, in these cases the electrolyte compositions and temperatures may be as set forth in the following examples.

Example 11 Electrolyte 35 mole percent NaCl,

mole percent MgCl Temperature Above M. P. of 430 C.

} Example III Electrolyte 67 mole percent KCl, 33

mole percent MgCl Temperature Above M. P. of 430 C. preferably above 500 C. Example IV Electrolyte 35 mole percent BaCI 65 mole percent LiCl. Temperature Above M. P. of 510 C.

Example V Electrolyte mole percent SrCl 50 mole percent NaCl. Temperature Above M. P. of 565 C.

Referring now to Figs. 4, 5, and 6, there is shown a modification of the bipolar cell of Figs. 1 through 3 wherein each cathode 18a is formed of a hollow metallic member, to the inside 50 of which the titanium tetrachloride is introduced by pipe 30a. The hollow space 5 0 inside of the cathode 18a serves as a reduction zone in which the introduced titanium tetrachloride is reduced to titanium particles by the electrolyte metal liberated in the electrolysis zone adjacent that surface of the cathode 18a closest to the anode 16. This surface of the cathode is preferably provided with slotted members 52 which slant upwardly and inwardly to provide openings 53. The electrolyte metal generated on these slanting members 52. is guided through the openings 53 into the interior of the cathode where it reacts with the titanium tetrachloride vapor bubbling up inside of the cathode. The electrolyte returned to the cell is preferably introduced into space 50 by pipe 36a so as to provide lateral flow of the electrolyte within the cathode space 50. The curved top 54 for the cathode 18a serves to carry the vapors in the cathode space laterally to the end of the cathode Ida. In other respects, the operation and construction of the cell may be identical to that shown in Figs. 1 through 3.

In connection with the operation of the Figs. 4-6 embodiment of the invention, it should be pointed out that the circulation of the electrolyte may additionally include flow of electrolyte from the electrolysis zone 20 through the openings 53 in the cathode 1842. This allows the titanium to be carried back into the reduction zone 50 inside of the cathode.

While preferred embodiments of the invention have been described in considerable detail above, numerous modifications can be achieved without departing from the spirit of the invention. For example, the circulation of the electrolyte may be assisted by the use of an inert gas lift or by pumping the electrolyte out of and into the cell near the bottom thereof. This modification is quite useful when relatively low electrolyte velocities are encountered in the settling zones and the bottom of the cell. Such low velocities may permit too much collection of titanium fines, but this can be prevented by such additional stirring or circulation as mentioned above.

Since certain changes may be made in the above pr: s and apparatus without departing from the scope oi the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process for producing titanium which comprises the steps of providing a closed electrolytic cell having a plurality of bipolar electrodes to provide a plurality of cathodes and anodes, pairs of said cathodes and anodes defining electrolysis zones therebetween, providing in said cell an electrolyte bath composed of a molten mixture of halides from the group consisting of the alkali metal and the alkaline earth metal halides, introducing titanium tetrachloride below the surface of the electrolyte into the space between an anode and a cathode in said cell, said titanium tetrachloride being introduced in the form of fine gas bubbles adjacent the bottom of said cathode, electrolyzing said molten mixture by passing an electric current therethrough, circulating said electrolyte upwardly in each electrolysis zone and laterally into a settling zone in communication with said electrolysis zone, and freezing said electrolyte adjacent the edges of each settling zone, thereby electrically isolating said zones.

2. A process for producing titanium which comprises the steps of providing a closed electrolytic cell containing anelectrolyte bath composed of a molten mixture of halides from the group consisting of the alkali metal and the alkaline earth metal halides, providing within said cell a plurality of bipolar electrodes which provide a plurality of cathodes and anodes, pairs of said cathodes and anodes defining electrolysis zones therebetween, said zones being separated by frozen electrolyte barriers, introducing titanium tetrachloride below the surface of said electrolyte into the space between an anode and a cathode in said cell, said titanium tetrachloride being introduced in the form of fine gas bubbles adjacent the bottom of said cathode, electrolyzing said electrolyte by passing a current therethrough, circulating said electrolyte upwardly in each electrolysis zone by introducing said gas bubbles into the bottom and laterally into a settling zone in communication with said electrolysis zone by introducing said electrolyte into the side of said cell, and freezing said electrolyte adjacent the edges of each settling zone, thereby electrically isolating said zones.

3. In a process for the production of titanium within an electrolytic cell provided with an electrolyte bath composed of a molten mixture of chlorides from the group consisting of alkali metal and alkaline earth metals, an anode and a cathode forming an electrolysis zone essentially free of any mechanical restrictions to electrical currents passing therebetween, the steps which comprise introducing titanium tetrachloride in the form of fine bubbles below the surface of the electrolyte into the space between the cathode and the anode of said cell adjacent the bottom of said cathode, electrolyzing said electrolyte by passing a. current therethrough, circulating said electrolyte upward through the electrolysis zone between said cathode and anode and transversely into the collection zone adjacent the end of said electrolysis zone and then back to said electrolysis zone by introduction of said electrolyte 'into the electrolysis zone at a point opposite said collecting zone, thereby removing the titanium formed within said electrolysis zone to said collection zone wherein the larger particles of said titanium are separated out for collection and the fine particles of said titanium remain suspended in said electrolyte and are rcturned to the electrolysis zone.

References Cited in the file of this patent UNITED STATES PATENTS 1,018,802 Acker Feb. 27, 1912 1,043,154 Seward et a1. Nov. 5, 1912 1,545,383 Ashcroft July 7, 1925 2,111,264 Gilbert Mar. 15, 1938 2,148,345 Freudenberg Feb. 21, 1939 2,432,431 MacMullin Dec. 9, 1947 2,607,674 Winter Aug. 19, 1952 2,741,588 Alpert et al Apr. 10, 1956 FOREIGN PATENTS 263,301 Germany Aug. 5, 1913 OTHER REFERENCES Cordner et 211.: Australian Journal of Applied Science, vol. 2 (1951), pp. 358-367. 

1. A PROCESS FOR PRODUCING TITANIUM WHICH COMPRISES THE STEPS OF PROVIDING A CLOSED ELECTROLYTIC CELL HAVING A PLURALITY OF BIPOLAR ELECTRODES TO PROVIDE A PLURALITY OF CATHODES AND ANODES, PAIRS OF SAID CATHODES, AND ANODES DEFINING ELECTROLYSIS ZONES THEREBETWEEN, PROVIDING IN SAID CALL AN ELECTROLYTE BATH COMPOSED OF A MOLTEN MIXTURE OF HALIDES FROM THE GROUP CONSISTING OF THE ALKALI METAL AND THE ALKALINE EARTH METAL HALIDES, INTRODUCING TITANIUM TETRACHLORIDE BELOW THE SURFACE OF THE ELECTROYLTE INTO THE SPACE BETWEEN THE ANODE AND A CATHODE IN SAID CELL, SAID TITANIUM TETRACHLORIDE BEING INTRODUCED IN THE FORM OF FINE GAS BUBBLES ADJACENT THE BOTTOM OF SAID CATHODE, ELECTROZING SAID MOLTEN MIXTURE BY PASSING AN ELECTRIC CURRENT THERETHROUGH, CIRCULATING SAID ELECTROLYTE UPWARDLY IN EACH ELECTROLYSIS ZONE AND LATERALLY INTO A SETTLING ZONE IN COMMUNICATION WITH SAID ELECTROLYSTIC ZONE, AND FREEZING SAID ELECTROLYTE ADJACENT THE EDGES OF EACH SETTLING ZONE, THEREBY ELECTROCALLY ISOLATING SAID ZONE. 